1. Technical Field
The invention relates to the exchange of information over an electronic network. More particularly, the invention relates to providing a redundant connection for devices and/or paths within an electronic network.
2. Description of the Prior Art
It is desirable to preserve network connectivity to a device or path in an electronic network. To this end, it is often preferred to provide a dual connection to network devices and/or paths where a second, redundant connection takes over if a primary connection fails. Such arrangement provides for greater network reliability.
Two broad approaches to providing such redundancy are presently known in the prior art:
One type of redundant connection concerns an Open Systems Interconnection (OSI) layer-1 (physical layer) implementation. This implementation is based on hardware signals which indicate whether or not the connection itself, and possibly a device at the other end, is functioning properly. If these signals cease, it is assumed that either the device or the cable link to the device have gone down, and that the redundant connection should be activated. An example of this solution involves using 10Base-T linkbeat (also known as linkpulse) to detect a working connection to a device which is powered-up.
While this type of solution can be implemented purely in hardware, and is thus inexpensive, it does not account for the possibility that network management may have been configured so that network communications cannot occur across the link, even when hardware signals are possible. For instance, if the device at the other end of the connection is configured for security and the device instigating the redundant connection does not match the permitted configuration on one or both of its connections, these offending links are essentially unusable. Moreover, the network administrator cannot be informed of this. In the case of such a non-permitted configuration, a redundant dual-connection does not really exist because one or both links cannot establish a connection.
The second type of redundant connection concerns the Open Systems Interconnection (OSI) layer-2 (MAC layer) implementation. This implementation is based on the exchange of network packets with a specific device, and requires both devices to have a network protocol stack. If the target device fails to respond to the packets, that device or its connecting cables are presumed to be inactive. Switching over to the redundant connection opens an alternate path to that target device. An example of this solution is the Backup Links feature provided on Hewlett-Packard Company's (Palo Alto, Calif.) 10Base-T hubs or, to a certain extent, the IEEE 802.1d Spanning Tree Protocol implemented in most standard network bridges.
This second solution can indicate whether the instigating device's immediate neighbor approves of its configuration but, to prevent network loops, the redundant connection cannot be tested without disrupting the main connection. The ability to respond to a trouble condition is also slower with this method than with the first method due to round-trip packet propagation time and the possibility of the target device being temporarily busy, which must be accounted for before a failure can be determined.
The IEEE 802.12 committee is currently in the balloting process for a Redundant Links feature definition to be included in the 802.12 standard for 100VG-AnyLAN networks. See Draft Standard, Information Technology, Local and Metropolitan Area Networks - Part 12: Demand-Priority Access Method, Physical Layer and Repeater Specifications for 100 Mb/s Operation - Redundant Links, Draft 3.0, LAN MAN Standards Committee, IEEE Computer Society, March 1996.
The redundancy feature, as described in the implementation proposed for the standard, is closer to the first type of Redundant Links solution (described above) than to the second. The proposed standard describes a general sequence of hardware meta-states through which a main and redundant uplink connection pass. There is freedom for individual implementers to decide the exact impetus for a transition from one meta-state to the next, and the severity of the conditions which cause such a transition.
In the proposed standard, redundant links are provided such that "both end nodes and repeater cascade ports may optionally be equipped with a second (redundant) link to maintain connectivity in case of individual link or repeater failure in the network connection path." (ibid. ) To better understand the environment in which the subject invention operates, as well as the limitations imposed by the prior art, a brief discussion of a typical network and device/path redundancy is now provided.
For an 802.12 repeater with redundant links, the purpose of such redundancy is to maintain network connectivity for all of a repeater's lower entities in the event of a failure in either of the repeater's uplinks or a connected higher-level repeater. FIG. 1 is a block schematic diagram of a topology that maintains network connectivity for any repeater experiencing a single uplink failure or failure to one connected higher-level repeater. The proposed standard provides that all redundant links shall be connected within the same arbitration domain and shall be connected in a manner that maintains a star topology, but does not result in a network loop for any combination of active links. One level-2 repeater is preferably designated as the alternate-root repeater. The alternate-root repeater preferably has both uplinks connected to local ports on the root repeater. All non-alternate-root, level-2 repeaters preferably have one uplink, designated as the primary uplink, connected to a local port on the root repeater, and the other uplink, designated as the secondary uplink, connected to a local port on the alternate-root repeater. All level-3 and lower repeaters preferably have their two uplinks connected to local ports on different repeaters at the next higher level.
As shown on FIG. 2, failure of the (primary) uplink between a level-2, non-alternate-root repeater and the root repeater can cause an extension of the root-repeater-to-lowest-end-node distance. It is therefore preferred that user implementation of this system ensures that the longest path distance between the root repeater and the lowest end node, including any added levels due to the level-2 standby link connection, does not exceed the maximum topology length defined in the standard. The primary uplink from a level-2, non-alternate repeater should be the active link whenever that link is operational.
When non-redundant-link repeaters are interconnected with redundant-link repeaters in the same fault-tolerant network, a repeater without redundant-uplink capability loses network connectivity if its uplink or the connected higher-level repeater fails. To minimize connectivity loss, non-redundant-link repeaters may be used as a root repeater, because root repeaters do not require a cascade port or any uplinks; or they may be connected at the lowest repeater level in a cascade. However, a non-redundant-link repeater is preferably not used as an intermediate repeater in a cascade. For example, with regard to FIG. 3, the failure of the root repeater or failure to the repeater 2a uplink results in loss of connectivity, not only for repeater 2a end nodes, but also for all lower repeaters and their end nodes in the cascade.
FIG. 4 is a block schematic diagram showing connections in a network for redundant links from end nodes. End nodes that are equipped with multiple MAC adapters may be connected to repeaters at any level in the network. However, to ensure maximum recoverability of network connectivity from single link or single repeater failures, alternate links from the same end node should be connected, such that the alternate network paths do not contain any instance where both paths use the same link or the same repeater. It is preferred that redundant end node links be connected to local ports on redundant link repeaters.
The network shown in FIG. 4 provides examples of both properly and improperly connected redundant-link end nodes. The alternate connections of end nodes x, y, and z have no common redundant path elements and can recover network connectivity in case of a single link or single repeater failure anywhere in the network. The alternate connection for end nodes r, s, and t each have common path elements and may not recover connectivity in the case of repeater or link failure. Thus, end node r loses most of its network connectivity if repeater 2c fails; end node s loses most of its network connectivity if repeater 3a's uplink fails, and all of its connectivity if repeater 3a, itself, fails; and end node t loses most of its network connectivity if repeater 2b fails.
While the proposed IEEE standard for 802.12 offers such redundancy, the proposed standard does not address such issues as storing and/or reporting conditions that lead up to a failure; accommodating minor, temporary disruptions; allowing status inquiries with regard to a redundant link while maintaining a primary connection; and continuing testing the redundant link after an initial verification. It would be advantageous to provide such enhancements to a network redundancy scheme, such as that set forth in the proposed standard.